1. Field
The present invention relates to systems and methods for implementing channel matched filter (CMF) circuits, and particularly for implementing channel matched filter circuits to facilitate spread spectrum reception in a wireless local area network (LAN). For example, the present invention finds application in the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard for wireless LANs using spread spectrum.
2. Description
In a typical wireless communications system, the transmitted signal travels through multiple propagation paths before arriving at the receiver. Along each path, components of the transmitted signal undergo random attenuation and phase offset, and may add destructively or constructively with each other at the receiver; this multipath effect can cause degradation of signal-to-noise ratio (SNR), resulting in severe performance impairment. Therefore, it is desirable to appropriately combine these multipath components such that the SNR is enhanced (relative to that of any individual multipath component). Implementing short packet wireless data communications over a wireless channel, such as that specified in IEEE 802.11, poses challenges in combating these multipath problems.
The channel matched filter (CMF) is commonly used to maximize the received SNR in a technique sometimes referred to as “maximal ratio combining”. However, previous implementations of this technique assume correction of the channel effects at a chip level, i.e. before the despreader. This is generally done because of the need for time and phase tracking. Applying this approach, the signal component from each path is weighted by the complex-conjugate of the (complex-valued) channel gain of the corresponding path; the sum of these weighted components has the optimum SNR, and is passed to the remaining portion of the receiver (such as a decision device, or the decoder).
FIG. 1 is a block diagram of a conventional CMF for a communication system utilizing direct-sequence spread spectrum signaling. The conventional CMF 100 is implemented as a pre-filter 102 followed by a despreader 104, i.e. a “front-end” CMF, because the pre-filter precedes the despreader. The sampling period is denoted as Ts and the complex valued channel gain of the ith multipath component as h[i], where h[i] is the discrete-time version of the continuous-time channel impulse response (CIR) h(t), i.e., h[i]=h(t)|t=iTs. The pre-filter has a response of ĥ*[−i], i.e. matched to the time-reversal and complex-conjugated version of the CIR, where * denotes the complex conjugation operation, and ĥ[i] is an estimate of h[i].
In the conventional CMF 100, the received signal samples are first convolved with the pre-filter 102, ĥ*[−i], which performs “matched filtering” to generate the combined samples with enhanced SNR. The despreader 104 correlates these combined samples of the pre-filter 102 that correspond to a symbol interval (indicated by the symbol rate switch 106) to generate the decision variable. The decision variable output 108 can be used to produce an estimate of the transmitted symbol and/or to drive the synchronization loops. Implementation of the CMF 100 in this manner requires complex DSP or microprocessor computations.
There is a need for channel matched filters with improved speed and efficiency. There is particularly a need for channel matched filters which do not require complex processing and computation. Further, there is a need for such channel matched filters in WLAN applications. The present invention meets these needs.